Organic light-emitting transistors with an efficiency that outperforms the equivalent light-emitting diodes

Organic light-emitting transistors with an efficiency that outperforms the...

(Parte 2 de 3)

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Figure 3 | Optoelectronic characteristics of the trilayer OLET and topographical images of the individual layers forming the heterostructure. a,b, Locus electrical curves of the OLET in n-polarization (a) and in p-polarization (b). During the n-polarization the electroluminescence output power (magenta) is also collected. c, In the transfer characteristic curves, the source–drain current (IDS) is measured keeping the drain–source potential constant at 90V, while sweeping the gate-source potential from 0 to 90V. d, AFM image of a 7-nm-thick DFH-4T film grown on glass/ITO/PMMA substrate. e, AFM image of a

40-nm-thick film of Alq3:(3%)DCM blend grown on top of the DFH-4T thin film reported in d. f, AFM image of a 15-nm-thick DH-4T film grown on top of the Alq3:(3%)DCM film reported in e. For ease of comparison the same z-axis colour scale is used for both images e and f.

emitted photons and the flowing charges (OLET drain current), thereby avoiding any risks of overestimating the EQE.

To verify whether improved interfacial characteristics would increase the efficiency, we fabricated a trilayer OLET with the order of the two charge transport layers reversed. Indeed the flat morphology of the DH-4T layer might be favourable for the realization of a smoother top interface in the heterostructure. The top electrodes were made of LiF/Al to favour electron injection into the LUMO level of DFH-4T. By implementing the DH-4T thin film (7nm) as a first layer in contact with the PMMA and the DFH-4T thin film (25nm) as a top layer, a maximum EQE exceeding 5% and a symmetric EQE profile peaking at the position of maximum ambipolarity (Fig. 5b) is obtained. The impressive improvement of the EQE obtained by controlling the interfaces in the heterostructure demonstrates the potential of our approach. Note that the current density of this reverse device configuration

ARTICLES NATUREMATERIALSDOI:10.1038/NMAT2751 Drain

Drain Drain

Drain

VGS = 0 VVGS = +30 V VDS = +90 V

VDS = +90 V VGS = +90 V VDS = +90 V VGS = +60 V

VDS = 0 V 10 µm c d b

Figure 4 | Images of the light-emitting area within the OLET device channel. a, For reference, an optical micrograph of the device channel without bias, to highlight the position of the drain electrode edge that is marked with a yellow line. b–d, Optical micrographs of the emission zone within the device channel of the trilayer heterostructure OLET during a transfer scan at VDS =90V and VGS values of 30V (b), 60V (c) and 90V (d). Three arrows in b–d indicate the initial position of the recombination and emission zone.

ITOGlass ITO

Au Au

Glass

DH-4T Al/LiF

Alq3:DCM 3% DHF-4T

Log (

Log (

Al/LiF DHF-4T

Alq3:DCM 3% DH-4T a b

Figure 5 | EQE as a function of the applied gate voltage for the two trilayer heterostructure OLET configurations. a, The bottom layer and the top layer are thin films of DHF-4T and DH-4T, respectively. b, The layer configuration of a is reversed. The transfer curves with the drain–source current (IDS) plotted on a logarithmic scale are also reported. IDS is measured keeping the drain-source potential constant at 90V, while sweeping the gate-source potential from 0 to 90V.

is decreased by a factor of 10 with respect to the previous case, as a result of the large threshold voltage (>|50| V) for both charge types. However, our data demonstrate that the achievement of balanced ambipolar transport in devices with current densities similar to those observed in the first trilayer OLET, would enable OLET devices with simultaneous high efficiency and brightness.

To enable a direct OLED versus OLET experimental comparison we fabricated the device schematized in Fig. 6a, with the aim of implementing the OLET trilayer active region in an equivalent OLED structure. The layer sequence, thickness and film growth parameters are exactly those used for the OLET fabrication and the electrodes are ITO (coated with a poly(3,4- ethylenedioxythiophene) (PEDOT) layer, anode) and Au (cathode). As the workfunctions of ITO/PEDOT and Au are similar, the charge injection conditions in the OLED configuration mimics the OLET case where both the drain and source electrodes are made of gold. The optoelectronic characteristics of this device (Fig. 6b) follow a typical L–I–V OLED behaviour and exhibit a maximum EQE

NATUREMATERIALSDOI:10.1038/NMAT2751 ARTICLES DH-4T

Glass Glass

Au (+)

Alq:DCM 3%

DHF-4T PEDOT

Al/LiF (¬) DHF-4T

Alq:DCM 3%

DH-4T

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1.2 × 10 1.0 × 10

8.0 × 10 6.0 × 10 4.0 × 10

2.0 × 100 2 × 10

1 × 10 8 × 10 6 × 10 4 × 10

Figure 6 | Device structure and optoelectronic characteristics of the trilayer OLED in direct and reverse configurations. a, Schematic structure of the trilayer OLED in the direct configuration. b, Optoelectronic characteristics of the OLED sketched in a. c, EQE of the direct heterostructure OLED. d, Schematic structure of the trilayer OLED in the reverse configuration. e, Optoelectronic characteristics of the OLED sketched in d. f, EQE of the reverse heterostructure OLED.

of ∼0.012% (Fig. 6c). Therefore, the control over the quenching and loss mechanisms results in an organic electroluminescencegenerating device with two orders of magnitude higher efficiency. Furthermore, we fabricated an OLED with the structure

ITO/PEDOT/DH-4T/Alq3:DCM/DFH-4T/LiF/Al. In this reverse configuration, hole injection from ITO into the DH-4T hole transport layer, and electron injection from Al into the DFH-4T electron transport layer are optimized. The EQE of this OLED is <0.01%(seeFig. 6f),whichisabout500timeslowerthanthatofthe corresponding OLET. Finally, the comparison of our OLET with a thoroughly optimized OLED based on Alq3:DCM as the emitting layer34, provides an important figure of merit to fully appreciate the advantages of the trilayer OLET configuration with respect to conventional OLEDs. Note that the reported EQE of an optimized OLEDbasedonthesameemittinglayeris2.2%(ref.34).Allofthese data clearly indicate that the quenching and loss processes in our trilayer OLET devices are minimized.

(Parte 2 de 3)

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